This invention relates to a multilayer wiring board allowing interlayer connection with a bump and to a method for manufacturing the same, and more particularly to a new multilayer wiring board ensuring reliability in connection as well as in junction and to a method for manufacturing the same.
To manufacture a so-called build-up multilayer wiring board, it is necessary to laminate insulating layers and conductive layers one by one to pattern each of the conductive layers to a predetermined wiring pattern and to facilitate interlayer connection between the conductive layers. Consequently, technologies for forming a fine pattern on the conductive layers and for achieving effective interlayer connection become more important.
As a method for manufacturing the build-up multilayer wiring board, such a method has been known that bumps, which are provided to a copper foil, are buried in the insulating layer and thereafter the other copper foil is cemented onto the insulating layer to connect to the bumps (see, e.g., Patent Document 1).
The invention described in Patent Document 1 relates to a selective etching method and a selective etching apparatus both for forming the bumps. Patent Document 1 discloses technologies for manufacturing a multilayer wiring circuit board in which an etching barrier layer is provided to a main face of the copper foil to be provided with the bump; a member used for forming a wiring circuit board, which is provided with a copper foil used for forming a conductive circuit, is used as a base on a main surface of the etching barrier; and this member is processed as appropriate to obtain the multilayer wiring circuit board.
Through the technologies for manufacturing the multilayer wiring circuit board, first, the foil copper of the member used for forming the wiring circuit board is selectively etched to form the bumps for interlayer connection, and the intervals between the bumps are filled by the insulating layer to insulate each of the intervals. Next, the copper foil used for forming the conductive circuit is formed on upper surfaces of the insulating layer and the bumps. Subsequently, wiring films are formed by selectively etching the foil coppers at both upper and lower surfaces. Consequently, the multilayer wiring board is formed, in which the wiring films are provided at both the upper and lower surfaces and the bumps connect the wiring films to each other.
Patent Document 1 is Japanese Patent Laid-Open Publication 2003-129259.
In the multilayer wiring board manufactured by the aforementioned technologies for manufacturing the multilayer wiring circuit board, since electronic connection between the copper foils and the bumps are achieved by a molding thermocompression bonding, ensuring reliability in connection becomes a big challenge. As described above, the insulating layer is formed after formation of the bumps, and the copper foil used for forming the conductive circuit is formed on the upper surfaces of the bumps and cemented onto the insulating layer by a thermocompression bonding. At this time, the copper foil is only in contact with top surfaces of the bumps, thus having a small binding force, so that there are many problems in terms of reliability in connection, such as poor connection caused by a slight external force.
In consideration of the situation described above, bump-connection (connection between the copper foil and the top surface of the bump) has been a high priority, so that the thermocompression bonding is performed in a so-called pure copper condition to minimize oxidation of the copper foil to be used. In view of the bump-connection, since formation of the oxide film on a surface of the copper foil has a disadvantage in conduction, it is more desirable to bring metals into contact with each other. Therefore, reduction of reliability in connection due to the oxide film is prevented by performing the thermocompression bonding immediately after removal of an oxide coating of the copper foil to be used.
However, as a result of earnest investigations made by the inventors, it turned out that the thermocompression bonding upon removal of the oxide coating of the surface of the copper foil results in lack of intimate contact of the cooper foil to the insulating layer, and thus junction strength of the copper foil subjected to the thermocompression bonding cannot be adequately achieved. In fact, it has been seen that a measured value of peel strength of the copper foil subjected to the thermocompression bonding is about 5 N/cm, which is far below required peel strength (about 6 to 10 N/cm). Lack of the junction strength as mentioned above causes accidental separation of the copper foil, thereby creating a more serious problem than lack of the aforementioned reliability in connection.